test: remove 3rd argument from assert.strictEqual

If there is an AssertionError, the string literal is printed and not the
value of `r`. For debugging purposes, it is good to know if `r` is false
or null or something else.

PR-URL: #19707
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-http-res-write-after-end.js

@@ -34,7 +34,8 @@ const server = http.Server(common.mustCall(function(req, res) {
   res.end();
 
   const r = res.write('This should raise an error.');
-  assert.strictEqual(r, true, 'write after end should return true');
+  // write after end should return true
+  assert.strictEqual(r, true);
 }));
 
 server.listen(0, function() {